Permanent Setting of Wool
Permanent setting takes place in wool because the disulphide bond cross links that stabilise the protein matrix can be rearranged under appropriate conditions of pH, temperature and humidity. The chemical basis for rearrangement of the disulphide cross links is the thiolate-disulphide exchange reaction. The rate at which the disulphide bonds rearrange depends on the temperature and the thiolate ion concentration. The thiolate ion concentration can vary with the previous history of the wool and the pH of the fabric. Chemically assisted permanent setting treatments all increase the rate of setting by raising the concentration of thiolate groups. The thiolate concentration in wool can be increased in two ways:                reaction with reducing agents,        increasing the pH.        
Hence, many setting agents combine both of these aspects. The use of after-treatments following chemical setting is highly desirable to stabilize the wool by inhibiting further thiol/disulphide interchange. This is achieved by oxidising any free thiol groups, and by re-establishing an acidic pH in the fibre.
In practice, the rate of permanent setting is determined by a number of variables such as conditions of temperature, regain, duration of time, pH etc. FIG. 1 shows the approximate conditions of temperature and regain that are required to achieve 50% permanent set within 10 minutes at pH 5.5, with untreated wool. These represent approximately minimum conditions for batch treatments.
Under practical conditions, permanent setting is always less than 100%. This is because stress relaxation is never complete. Complete relaxation is prevented by the rigidity of the protein crystals in the matrix, the inability of some of the crosslinks in wool to rearrange (e.g. lanthionine) and the introduction of more non-labile crosslinks while disulphide bond rearrangement is taking place.
During processing, permanent setting of distortions should be avoided whenever practically possible since it may not be possible to completely remove permanently set faults such as creases at a later stage. For example, running marks permanently set into fabric, during scouring or piece dyeing, may not be completely removed by any subsequent permanent setting processes.
The most important dimensional properties: relaxation shrinkage and hygral expansion, quantify changes in fabric dimensions in response to changes in the environment. Hygral expansion or “hygral behavior” is important mainly for the hydrophilic fibers and the largest and most important effects are found only with wool and cotton.
Shrink Proofing of Wool
Wool can be made shrink resistant in two different ways.    1. The surface scales on the fibres can be modified or    2. The fibres can be bonded together using a polymer.
The modification of the woolen fibers can be achieved as following:                Oxidation with chlorine (in the form of chlorination) in aqueous treatments causes the scales to be partly dissolved and they loose their ability to tangle together by the ratcheting mechanism and hence the surface scales of the fiber get modified.        If an adherent polymer is applied by a pad-dry procedure, a high proportion of the fibers become bonded together at a few points along their length. Thus the fibers are immobilised and cannot migrate and felt together.        
The chlorination process involves degradation of the fiber and overall weight is lost, whereas with the resin/polymer process, weight is gained since it is an additive process. Sometimes chlorinated wool is treated with a resin by padding or exhaustion to compensate for the weight loss and suitable resins also increase the shrink resistance to some extent.
Chlorination is almost always the treatment of choice as a preparatory step before printing on wool. In this case a polymer is usually not applied. Treatment levels are equivalent to 1-4% o.w.f. of active chlorine. Chlorination treatments can have several adverse effects, both on the wool fabrics (for example yellowing and harsh handle) and on the environment.